1. Technical Field
The present invention relates to a three-phase electric furnace comprising heating elements connected to each phase. More particularly, the present invention relates to a method of controlling the heating power generated by the furnace and by each heating element during a heating process. The invention also relates to three-phase electric furnaces which are specifically utilized for sintering cemented carbide blanks.
2. Technical Background and State of the Art
Cemented carbide bodies are produced by powder metallurgical techniques including wet mixing of powders to form the constituents of the bodies; drying the milled mixture to a powder, generally by spray drying; pressing the dried powder into bodies having a desired shape; and sintering.
Sintering is performed in large furnaces which have a total volume of about 2 m.sup.3. These furnaces include a furnace cavity which accounts for about 10 percent of the furnace's total volume. The sintering temperature is 1440.degree.-1500.degree. C., and it is very important that the sintering furnace be capable of maintaining a constant temperature between the different zones within the furnace, for example, a zone-to-zone difference that does not exceed .+-.5.degree. C. This is especially important when producing modern cemented carbide grades which often have highly complex structures.
In prior designs, sintering furnaces employ power supplies which comprise a three-phase transformer. The primary side of the three-phase transformer is connected to a power source via a current regulator, while each of three heating elements is connected to a respective phase on the secondary side of the transformer. The temperature inside the furnace cavity is measured in one place by a temperature sensor which is, in turn, connected to the current regulator. Using the temperature information provided by the temperature sensor, the current regulator corrects the electric current in each phase using phase angle control. The current regulator is capable of making the corrections in parallel. However, sintering furnaces employing this type of temperature control scheme do not and cannot take the zone-to-zone temperature differentials, that exist within the furnace cavity, into consideration.
At extremely high temperatures, graphite rods are used as heating elements. Graphite rods require a supply voltage that is lower than the voltage of the power source and this is the reason why the transformer is necessary. The graphite rods are connected in such a way that they create a star-connected load without a neutral wire. This means that the furnace only has three lead-throughs into the furnace cavity for the respective phase conductors. When using heating elements that require higher supply voltages (e.g., elements of tungsten), the transformer may be omitted.
From one zone to another zone within the furnace cavity, temperature differentials may arise for several reasons. For example, the amount of cemented carbide blanks may vary at different locations within the furnace; the insulation of the furnace may change during the life of the furnace, resulting in large heat leakages at certain places in the furnace; and the phase voltages driving the respective heating elements may vary. Correcting the problems associated with these temperature differentials can be accomplished by individually controlling the heating elements.
The power generated by a furnace employing star-connected graphite rod heating elements is on the order of 200 kVA; the phase voltage supplied to the graphite rods is on the order of 50 V; and the phase currents through the graphite rods may reach 2.5 to 3.0 kA. The construction and location of the graphite elements in the furnace cavity are well suited for controlling the power within three zones.
Therefore, one object of the invention is to provide a method that individually controls the power generated by each heating element in a three-phase, electric furnace.
Another object of the invention is to provide a simple, cost-effective power control system that can be utilized with existing furnaces and new sintering furnaces (i.e., furnaces without a neural wire).